Chapter1, Genes, Genetic Codes and Mutation NUCLEOTIDE SEQUENCES DNA - two complementary strands 4 kinds of nucleotides - 2 purines [R] (adenine [A], guanine [G]), 2 pyrimidines [Y] (thymine [T], cytosine[C]) A=T weak bond, G≡C strong bond, canonical base pairs 5' - upstream, 3' - downstream heavy strand - contains many purines, light strand - contains many pyrimidines RNA -..